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Functional Electrical Stimulation Research Overview

The core of this research is synthesis of the arm free standing in complete thoracic spinal cord injured subjects.

The maintenance of upright posture is such a common occurrence among human beings that is perhaps the most universally accepted measure of normality. The ability to control the upright posture is not only of significant importance for enabling the subject to perform functional tasks but also enables the beginning and the termination of a walking sequence. Arm free standing would be required for the paralyzed subject to be able to perform some daily or professional tasks. However, the present state of the art in the rehabilitation based on the functional electrical stimulation requires use of arms for providing support and balance during standing.

From the biomechanical and the control point of view, the human body is an multisegment unstable system with time varying characteristics due to the movement of segments according to the task requirements. During the arm free standing the control system has to maintain the position of the body`s center of gravity over a relatively small supporting surface determined by the size and the position of the feet. Due to the complexity of the biomechanical system with numerous degrees of freedom, nonlinear and time varying nature of electrically activated muscles, multiple and partially functionally redundant nature of actuators, the interaction of biomechanical subsystems through force-producing interconnections, the existence of additional, possibly competing controllers (spinal reflexes, voluntary limb and body motions) this study focuses on the balance control in the sagittal plane.

The project considers the development of a novel control strategy for a dynamic closed-loop control of functional electrical stimulation of ankle joint antagonist muscles for restoration of unsupported standing in spinal cord injured subjects. The developed algorithm integrates the preserved upper body motor and sensor functions with the artificial control of the paralyzed ankle joints and ensures a stable standing of a paraplegic subject constrained in the mechanical rotating frame that limits the motion to the sagittal plane. The designed control system implicitly detects the subject`s volition through the observation of the voluntary activity of the subject`s upper body. The detected volition presents the reference for the control of the standing posture, based on the minimization of the ankle joint muscles effort. The proposed control does not allow autonomous standing of a paraplegic person yet, however it enables the stabilization of posture in one significant plane, thus lying the foundation for the achievement of the final goal.

The research is divided into three main sections. In order to assure control of balance without the arm support, first the ankle joints being far from their anatomical constrains of dorsiflexion and plantarflexion during standing, need to be stabilized. Therefore, the first section considers the synthesis of a novel ankle joint torque controller based on variable structure control systems theory. The second part of the study concerns the design of the balance control during arm free standing that integrates subject`s voluntary activity with the artificial control of ankle joints and ensures stable standing with the minimal required effort in the ankle joint muscles. In the third section, the control of arm free standing is synthesized based on the integration of the ankle joint torque and body balance controllers. Analysis of the arm free standing is performed.